This is one of those questions where the
answer presupposes the
question.
If this
planet hadn't been so
hospitable to the development of
carbon,
oxygen and
water reliant life, we wouldn't be here to ask the question in
the first place. Hence there are no
human beings on
Mercury saying
"Why is this
planet so inhospitable to our kind of
lifeform?".
The question it does raise however is just how perilous and unique
are the conditions that exist on earth, and could life develop under
other circumstances. My own personal view is that there is undoubtedly
life elsewhere in our universe: after all, even if the odds of life
developing here are a million to one against, or even a billion to one
against, the fact that
there are more stars in the universe than grains of sand on earth means that
single-cellular organisms or even basic protein chains will have
developed somewhere; whether any of them ever evolved beyond that point is
the real question.